The present invention is directed to a method of forming a pressure free expansion member with segments displaceable radially outwardly into an undercut in a borehole.
In the state-of-the-art there are two basic fastening techniques for providing pressure free expansion members, one is a bonded anchor, the other is an anchor member with expanding elements for forming a positive connection. An undercut dowel is generally formed of an axially extending sleeve with an axial bore therethrough and expanding elements which can be displaced radially and a threaded rod connected to a threaded expanding member, with the rod extending through the axial bore and connected to the expanding member at the front end of the bore. The segments are displaced radially outwardly by drawing the expanding member with a controlled force via the threaded rod into the sleeve. In place of a threaded rod with an expanding member, a conical bolt can be used and positioned in the bore so that its conical front end can be drawn into the bore. As a result, the expansion elements are displaced radially outwardly by driving the sleeve over the conical expansion. To form the anchorage, a cylindrical borehole is generally drilled in a base material in which the anchorage is to be formed. An undercut is produced at a specific depth in the borehole with suitable devices known in the art. Next, the undercut dowel is inserted into the borehole. The segments to be displaced are positioned in the region of the undercut. When the dowel is anchored, the segments are displaced radially outwardly into the undercut and a pressure free positive connection is effected. The receiving material for the anchorage is thus put under stress only when the dowel is prestressed or when it is engaged by an external load.
Anchorages with undercut dowels are used especially when heavy loads are to be secured. Because of the positive connection, the absence of expansion pressure, small axial and edge distances are possible. Undercut dowel systems have an adequate load carrying capacity even when the base material for the anchorage is cracked.
The second type of expansion free anchors are the bonded anchors. Bonded anchors include an anchor rod inserted into a borehole previously filled with an organic or inorganic bonding mortar or material. As a rule, a bonded anchor has an anchor rod with a point like leading end and a mortar cartridge to set the bonded anchor. First a borehole is provided in the base material forming the anchorage and, subsequently, a mortar cartridge with a sufficient amount of mortar is pushed into the borehole. During the setting procedure of the bonded anchor, the cartridge is destroyed, for example, by the pointed leading end of the anchor rod. The anchor rod is driven in by a hammer drill with a pulsating rotation movement. At the same time, the cartridge, usually formed a glass, is completely shattered. A mortar of high strength, usually setting in a relatively short time, is produced from the consolidated components of the mortar cartridge.
Bonded anchors are distinguished by the absence of expansion pressure and, therefore, can be used with smaller axial and edge distances than conventional anchors. The holding force of a bonding anchor is achieved by an adhesive bond between the mortar and the wall of the borehole and of the anchor rod which is sufficiently high. Bonded anchors are, therefore, also used for heavy load fastenings. Bonded anchors are particularly advantageous for fastenings in damp receiving materials forming the anchorage, for example, in a tunnel and in harbor installations.